1. Field of the Invention
The invention relates to nano-carbon materials, and more particularly to a method for functionalizing magnetic nano-carbon materials.
2. Description of the Related Art
Nano-carbon materials can be classified as carbon nanotubes, carbon nanofibers, and carbon nanocapsules. Carbon nanotubes comprise hollow tubes, the walls of which are pure carbon with a honeycombed structure similar to graphite. These structures can be multi-walled (MWNTs) or single-walled (SWNTs). Carbon nanocapsules contain multiple graphite layers forming a small spherical capsule inside a larger spherical capsule, making up polyhedral carbon groups. Diameters are around 3˜100 nm and outer layers contain the same graphite layer structure as MWNTs. Carbon nanocapsules are also classified into hollow carbon capsules and metal-filled carbon capsules filled with metals, metal oxides, metal carbides, or alloy materials.
Novel nano-carbon materials have potential application in catalysis, polymer composite, and biomedical fields etc. Applications of the nano-carbon materials mainly depend on the degree of the dispersion of the nano-carbon materials into the related medium, such as aqueous phase, organic phase, or polymer materials. Improvements in heat transfer, electrical properties, viscosity, and lubricity, etc. can be realized upon dispersion of the nanotubes. Control of the surface functional groups in the nano-carbon materials is thus important. For example, common nano-carbon materials exhibit hydrophobic properties making it difficult to disperse them in aqueous solution, requiring modification of surface functional groups thereon. To change the surface properties of the nano-carbon materials from hydrophilics to hydrophobics, conventional methods utilize oxidization by strong oxidants or acidification in the strong acid environment. U.S. Pat. No. 5,861,454 discloses the oxidation of fibrils with concentrated nitric acid resulting in a mass, which was difficult to disperse. Hiura and Ebbesen, in U.S. Pat. No. 5,698,175, describe a process for purifying and functionalizing carbon nanotubes, which after synthesis contain carbon impurities such as carbon nanoparticles and amorphous carbons. In this process, the nanotubes are dispersed with ultrasound into nitric acid, chlorosulfonic acid or potassium permanganate in dilute sulfuric acid solution and heated to purify the nanotubes or introduce functional groups into the nanotubes. U.S. Pat. No. 6,099,965 discloses the use of nitric acid (HNO3) to modify the surface functional group on the nano-carbon materials. There are drawbacks associated with the methods now available to provide oxidized carbon nanomaterials. For example, one disadvantage of using strong acid treatment is the generation of environmentally harmful wastes. Treating such wastes increases the cost of the products. Since the condition of acidification is uncontrollable, it is hard to do mass production. The use of strong acid such as nitric acid and sulfuric acid leads to corrosion problems. It would therefore be desirable to provide methods of functionalizing carbon nanotubes which do not use or generate environmentally hazardous chemicals and which can be scaled up easily and inexpensively. While many uses have been found for carbon nanomaterials, as described in the patents and patent applications referred to above, many different and important uses may still be developed if the carbon nanomaterial surfaces can be easily and inexpensively functionalized, permitting interaction of the functionalized carbon nanomaterials with various substrates to form unique compositions of matter with unique properties.
Conventional methods for preparing hydrophilic nano-carbon materials by oxidization or acidification are further limited by unsuitability for magnetic nano-carbon materials. Novel nano-carbon materials such as carbon nanotubes (CNT) and carbon nanocapsules (CNC) filled with metal particles have been developed to apply not only to the separation of the biomedical DNA, RNA, and active enzymes but also to recycling the heavy metal catalysis in the chemical industry. The metal particles of CNT and CNC, however, dissolve in acid, rendering the nano-carbon material to be nonmagnetic.
Therefore, in order that the magnetism of nano-carbon materials is remained, a novel method for functionalizing nano-carbon materials is called for.